In recent years, transmission capacity of a communication channel has been increased in a mobile communication network too. In the mobile communication network, not only text data and HTTP data as in an e-mail but also data whose amount is large such as picture or movie are transmitted.
It is considered that data whose amount is large and which is required to transmit in real time will be bidirectionally transmitted for applications such as IP telephone (VoIP) and videoconference in the future.
For example, a high speed communication network system using 1xEVDO (1x Evolution Data Only) system which is a mobile communication system dedicated to data communications is proposed.
In the system, a wireless base station transmits packets to each communication terminal being in the cover area of the wireless base station. Each communication terminal measures link quality of a downlink (for example, CIR (Carrier-to-Interference Ratio)) based on a pilot signal involved in reception packet.
Each communication terminal selects a communication mode wherein the communication terminal enables to perform high speed communications most efficiently in the measured link quality. Each communication terminal further transmits DRC information (Data Rate Control signal) representing the selected communication mode to the wireless base station.
The wireless base station refers to the DRC information transmitted from each communication terminal and allocates communication resources preferentially to the communication terminals with the better link quality.
Accordingly, since data is transmitted to the communication terminal with good link quality at a high transmission rate, required time for the communication can be shortened. Since data is transmitted to the communication terminal with poor link quality at a low transmission rate, error resilient can be enhanced.
The transmission rate of uplink (from wireless communication terminal to wireless base station) in the 1xEVDO system is determined based on the state of the wireless communication terminal and the wireless base station and is controlled.
That is, the transmission rate of uplink in 1xEVDO is selected from among 9.6 kbps, 19.2 kbps, 38.4 kbps, 76.8 kbps, and 153.6 kbps. The terminal sets the transmission rate of uplink to the lowest value 9.6 kbps at the communication start time.
This value is updated according to the maximum transmission rate information in broadcast information sent from the wireless base station.
That is, the transmission rate of the wireless communication terminal is set to 9.6 kbps in accordance with the maximum transmission rate just after the operation is started after power of the wireless communication terminal is turned on.
After communications are started, the wireless communication terminal performs an update test of the transmission rate at a given period within the range of the maximum transmission rate information sent from the wireless base station.
If the currently communicating wireless base station permits an increase in the transmission rate, the current transmission rate is maintained or can make a transition to one-higher step according to the result of the transmission rate update test.
On the other hand, if the currently communicating wireless base station does not permit an increase in the transmission rate, the current transmission rate is maintained or can make a transition to one lower step according to the result of the transmission rate update test.
FIG. 9 shows an example wherein the transmission rate of the wireless communication terminal is changed in accordance with the algorithm described above.
In the transmission rate in the related art shown in FIG. 9, the maximum transmission rate of the wireless communication terminal is set to 9.6 kbps at the communication start time. Therefore, the uplink transmission rate is also 9.6 kbps.
Thereafter, the wireless communication terminal receives the broadcast information sent from the wireless base station, and the maximum transmission rate of the wireless communication terminal is updated to 153.6 kbps. But, the current transmission rate is maintained until the next update timing.
If the wireless communication terminal succeeds in increasing the transmission rate at the update timing, the transmission rate of the wireless communication terminal becomes 19.2 kbps which is the next transmission rate step to 9.6 kbps.
Then, if the wireless communication terminal fails increasing the transmission rate, the current transmission rate is maintained. If the wireless communication terminal succeeds in increasing the transmission rate, the transmission rate makes a transition to one higher step.
Thus, if the wireless base station permits a high transmission rate, the uplink transmission rate increases only stepwise.
Patent document 1: JP-A-2002-171213
However, in the determination method of the uplink transmission rate described above, the communications are started at the lowest transmission rate and the transmission rate is increased stepwise.
This system does not involve any problem in use of application wherein the amount of data transmitted on the uplink is small such as WEB browsing or FTP downloading.
However, in application wherein a large amount of data is transmitted and a constant transmission rate is required also on the uplink such as IP telephone (VoIP) or videoconference, it takes time that the uplink transmission rate reaches a speed required for the operation of the application. Thus, it takes time that the application can be used just after starting communications.
For example, as shown in FIG. 9, if the operation of the application requires the transmission rate 64 kbps or higher, the application cannot be used until the transmission rate becomes 64 kbps or higher (arrives at 76.8 kbps) after connection is established.
FIG. 10 shows a data communication sequence in the communication system of the related art.
When a connection request is issued from an application (App) installed in a wireless communication terminal to the wireless communication terminal (AT: Access Terminal), the connection request is sent from the wireless communication terminal to a wireless base station (AP: Access Point).
Then, after a wireless communication line is established between the wireless communication terminal and the wireless base station, the application transmits data to a server (Serv).
However, since the initial value of the uplink transmission rate of the wireless communication line between the wireless communication terminal and the wireless base station is set to 9.6 kbps, the data transmitted from the application via the wireless communication terminal and the wireless base station cannot be decoded in the server. Therefore, the application cannot normally operate.
Thereafter, even if the uplink transmission rate increases to 19.2 kbps and 38.4 kbps, the application requiring the transmission rate 64 kbps or higher cannot normally operate at the transmission rate 19.2 kbps, 38.4 kbps.
Then, when the uplink transmission rate reaches 76.8 kbps, in the application requiring the transmission rate 64 kbps or higher, the data transmitted from the application can be decoded in the server. Therefore, the application starts to operate normally.
Particularly, in the 1xEVDO system, maintaining the current transmission rate or making a transition to one higher step is selected as probability according to the result of the transmission rate update test.
Consequently, a time of about 10 seconds may be required by the time the transmission rate increases from the initial value 9.6 kbps to 76.8 kbps depending on the result of the transmission rate update test. That is, time is required by the time the application starts to operate.